There are various Regulatory Bodies at the international and National level, which lay down norms for radiation protection. These are the International Commission for Radiation Protection (ICRP) the National Commission for Radiation Protection (NCRP) in America, and the Atomic Energy Regulatory Board (AERB) in India. These bodies recommend norms for permissible doses of radiation from X ray tubes and the shielding required for the walls of an X ray room. Data is also available from the work of Investigators regarding the room shielding required in a CT suite. The recommended lead equivalent in shielding apparel to be worm by radiation workers is 0.5 mm. The regulatory bodies also lay down safe dose limits for radiation workers and for the general public. The duties of the Radiation Safety Officer (RSO) are also specified by the regulatory bodies, as are the radiation surveillance and radiation safety programmers.

In our earlier article we have elaborated the biological hazards of radiation and the radiation doses which lead to these effects. In this article we introduce to the reader the various regulatory bodies especially the Indian Regulatory Body (AERB) and also the role of Radiation Safety Officer (RSO). We also appraise the reader of the objectives of radiation protection, the principles, methods and practices of radiation protection and the safe dose limits.

The Regulatory Bodies

The Regulatory bodies lay down norms for protection against radiation and also recommend the dose limits for radiation workers and the general public. The ICRP or the International Commission for radiation protection is the international regulatory body. Each country has its national counterpart of the ICRP. In America the counterpart is the NCRP or The National Commission for Radiological Protection and in India it is the AERB or the Atomic Energy

Regulatory Board.

The International Commission of Radiation Protection (ICRP) was formed in 1928 on the recommendation of the first International Congress of Radiology in 1925. The commission consists of 12 members and a chairman and a secretary who are chosen from across the world based on their expertise. The first International Congress also initiated the birth of the ICRU or the International Commission on Radiation Units and measurements [1].

The Indian regulatory board is the AERB, Atomic Energy Regulatory Board. The Atomic Energy Regulatory Board was constituted on November 15, 1983 by the President of India by exercising the powers conferred by Section 27 of the Atomic Energy Act, 1962 (33 of 1962) to carry out certain regulatory and safety functions under the Act. The regulatory authority of AERB is derived from the rules and notifications promulgated under the Atomic Energy Act, 1962 and the Environmental (Protection) Act, 1986. The mission of the Board is to ensure that the use of ionizing radiation and nuclear energy in India does not cause undue risk to health and environment. Currently, the Board consists of a full-time Chairman, an ex-officio Member, three part-time Members and a Secretary [2].

Objectives of Radiation protection

The ICRP in 1991 stated that "the overall objective of radiation protection is to provide an appropriate standard of protection for man without unduly limiting the beneficial practices giving rise to radiation exposure". The NCRP (1993), issued a similar statement in its Report (No. 116) that "the goal of radiation protection is to prevent the occurrence of serious radiation induced conditions (acute and chronic deterministic effects) in exposed persons and to reduce stochastic effects in exposed persons to a degree that is acceptable in relation to the benefits to the individual and to society from the activities that generate such exposure" [1]. Furthermore, the ICRP suggested that "current standards of protection are meant to prevent occurrence of deterministic effects by keeping doses below relevant thresholds and ensure that all reasonable steps are taken to reduce induction of stochastic effects"[1].

Radiation safety act in India

Radiation safety in handling of radiation generating equipment is governed by section 17 of the Atomic Energy Act, 1962, and the Radiation Protection Rules (RPR), G.S.R. - 1601, 1971 issued under the Act. The "Radiation Surveillance Procedures of Medical Applications of Radiation, G.S.R. - 388, 1989", issued under rule 15 specify general requirements for ensuring radiation protection in installation and handling of X-ray equipment. Guidance and practical aspects on implementing the requirements of this Code are provided in revised documents issued by AERB in the year 2001 [2].

Role of AERB (India)

AERB of India recommends and lays down guidelines regarding the specifications of medical X-ray equipment, for the room layout of X-ray installation, regarding the work practices in X-ray department, the protective devices and also the responsibilities of the radiation personnel, employer and Radiation Safety Officer (RSO). AERB is the authority in India which exercises a regulatory control on the approval of new models of X-ray equipment and the layout of any new proposed X-ray installation. It also is the regulatory authority for registration and commissioning of new X-ray equipment, inspection and decommissioning of X-ray installation, certification of a RSO and of service engineers and also for imposing penalties on any person contravening these rules [2].

Understanding the potential risks and benefits of exposure and making a benefit risk analysis

Before undertaking any radiological examination, it is important that the physician, radiologist and technologist all understand the potential risks of radiation and also its advantages or benefits to the patients. The potential risks of radiation have been explained in the earlier article as comprising of stochastic (of which probability increases with dose) and deterministic (of which severity increases with dose). Cancer induction and genetic effects are stochastic effects and cataracts, blood dyscrasias and impaired fertility are examples of deterministic effects.

On the other hand, the benefits of diagnostic radiology in orthopedic, gastrointestinal and neurological disorders is well known. However since radiation exposure entails inherent risks of radiation effects, no decision to expose an individual can be undertaken without weighing benefits of exposure against potential risks, that is, making a benefit risk analysis. Examples of a high benefit to risk ratio are CT in brain hemorrhage or coronary angiography in cardiovascular disease. Screening mammography in asymptomatic women below 35 yrs. of age is considered to have a low benefit to risk ratio [1].

Principles of radiation protection

The current radiation protection standards are based on three general principles :-

a) Justification of a practice i.e. no practice involving exposures to radiation should be adopted unless it provides sufficient benefit to offset the detrimental effects of radiation.

b) Protection should be optimized in relation to the magnitude of doses, number of people exposed and also to optimize it for all social and economic strata of patients.

c) Dose limitation, on the other hand, deals with the idea of establishing annual dose limits for occupational exposures, public exposures, and exposures to the embryo and fetus [1].

Justification of procedure versus the net benefit:

Any new radiological technique is subject to this principle. For example CT studies of the cranium outperform conventional imaging techniques for all intracranial disorders at a comparable dose of radiation. Similarly it is well known now that MRI studies of the spine offer much more diagnostic information than myelography, and that too without any radiation hazard. Therefore myelography in the present context is not justified [1]. Similarly, as explained above the benefit to risk ratio is high for CT brain in cerebro vascular hemorrhage and low in screening mammography in women below 35 years. However since CT examinations are known to contribute a large dose to diagnostic radiation it is recommended that CT referrals should also be reviewed meticulously [3].

Optimization of protection and the ALARA Principle:

Optimization of protection can be achieved by optimizing the procedure to administer a radiation dose which is as low as reasonably achievable (ALARA ), so as to derive maximum diagnostic information with minimum discomfort to the patient [1]. ALARA and ORP are concepts of the ICRP and the NCRP. ORP stands for Optimization of Radiation Protection. The history of the ALARA concept is traced back to the Manhattan project of World War-II that radiation exposures be kept at lowest possible level. This means that all radiation exposures to patients and personnel are to be kept as low as possible while still obtaining the accurate diagnostic information needed from the procedure. ALARA recognizes that there will always be some radiation exposure to patients involved in radiological procedures using ionizing radiation, but it also recognizes that these exposures can be minimized [1].

Judicious choice of investigations can significantly avoid not only radiation exposures but also increase both the diagnostic accuracy and working efficiency of a radiology department. These include substituting non-ionizing methods of examination in place of examinations involving ionizing radiation wherever possible; e.g. a combination of Ultrasound (US) and radio-nuclide study in place of Intravenous Urography (IVU), where US provides adequate structural information and radio-nuclide study provides the functional information. Evaluation of the lymph node status in the abdomen in various clinical conditions by US in place of repeated C.T. and use of Colour Doppler flow imaging in place of diagnostic angiography are two other simple methods to reduce radiation exposure to patients [3].

The greatest risk to the fetus of chromosomal abnormalities and subsequent mental retardation is between 8 and 15 weeks of pregnancy and examinations involving radiation to the fetus should be avoided during this period. For examinations which may involve rather heavy doses of radiation such as Barium enemas, pelvic or abdominal CT, the examination should be carried out during the first 10 days of the menstrual cycle to avoid irradiating any possible pregnancy [3].

Some of the methods to reduce radiation exposure, which show the maximum benefits of radiation protection and cause minimum extra costs, are also the simplest. These include avoiding repeat exposures by employing proper exposure factors, and maintaining a proper record of films so that repeat examinations can be avoided wherever possible [3].

In our opinion "optimization of protection" can be achieved by "optimization of the radiological procedure" so as to reduce radiation exposures to the minimum levels. This optimization is possible by good quality assurance and quality control. Factors which can contribute to dose reduction and quality assurance are, the use of high frequency three phase generator equipment, use of high KV technique and low mAs, (using the shortest exposure time), beam collimation and using proper beam filtration. The other factors which contribute to optimization of procedure are using a X-ray table top which allows high beam transmission, antiscatter grids, high speed films with rare earth screens, optimal film processing and largest possible source to image receptor distance (SID) [1].

High frequency generators help in generating high KV for use in the "high KV and low mAs technique". The high KV beam has higher energy photons, which undergo a lesser degree of beam attenuation and greater penetration of the beam through the patient. Therefore the tissue deposition of photons is reduced, which reduces the radiation dose to the patient [Figure1] [1].

Tube current or mA determines the quantity of the photons (in contradistinction to KV which determines the energy of photons) which also contribute to the patient dose. Increased exposure time also contributes to an increased patient dose. Since patient exposure is determined by exposure rate and time, the use of high frequency generators which generate high KV beam will decrease the mAs required to produce diagnostically adequate X-rays and hence reduce radiation dose to the patient.

Beam filtration causes lower energy photons to be absorbed by the filters thus increase the mean energy of the beam and its penetration power and hence decrease the patient dose. Beam collimation determines the size and shape of the beam. Field of view (FOV) which is the size of the irradiated area, directly effects the patient dose. Beam collimators control the FOV such, that only the diagnostically important area is irradiated. A good alignment between the field of the collimator and the radiation beam, reduces the possibility of beam cut off and beam overlap, thus reduces the chances of diagnostically inadequate exposures and prevents repeat exposures. The table top should be one which allows a high beam transmission which increases the quantity of diagnostically important photons reaching the film. A carbon fibre material is generally used for a high transmission table top. Antiscatter grids reduce scattered radiation reaching the film thus improving the quality of the resulting the radiograph and reducing chances of repeat exposures. Although antiscatter grids require slightly higher exposures and somewhat increased radiation dose to the patient, their advantage in producing improved quality of radiographs outweighs this minor risk of increased radiation dose to the patient [1]. Appropriate source to image receptor distance (SID) also has an important role in radiation protection, which is discussed in the subsequent section.

Correct exposure factors and technical considerations have a significant contribution towards patient shielding for CT examinations as well. For a CT examination of the skull, correct gantry angulation and beam collimation significantly reduced radiation dose to the orbits, despite the proximity of these organs to the x-ray beam, whereas the converse is true if correct technical considerations are not adhered to [3].

Radiation protection actions

The triad of radiation protection actions comprise of "time-distance-shielding". Reduction of exposure time, increasing distance from source, and shielding of patients and occupational workers have proven to be of great importance in protecting patients, personnel, and members of the public from the potential risks of radiation [1].

Time

The exposure time is related to radiation exposure and exposure rate (exposure per unit time) as follows :

Exposure time =

Exposure

Exposure rate

Or

Exposure = Exposure rate x Time

The algebraic expressions simply imply that if the exposure time is kept short, then the resulting dose to the individual is small [1].

Distance

The second radiation protection action relates to the distance between the source of radiation and the exposed individual. The exposure to the individual decrease inversely as the square of the distance. This is known as the inverse square law, which is stated mathematically as :

where I is the intensity of radiation and d is the distance between the radiation source and the exposed individual. For example, when the distance is doubled the exposure is reduced by a factor of four.

In mobile radiography, where there is no fixed protective control booth, the technologist should remain at least 2 m from the patient, the x-ray tube, and the primary beam during the exposure. In this respect, the ICRP (1982), as well as the NCRP (1989a), recommended that the length of the exposure cord on mobile radiographic units be at least 2 m long [1].

Another important consideration with respect to distance relates to the source-to-image receptor distance (SID). The appropriate SIDs for various examinations must always be maintained because an incorrect SID could mean a second exposure to the patient. Long SID results in less divergent beam and thus decreases the concentration of photons in the patients. Short SID results in the reverse action and increases the patient dose [Figure - 2] . Hence the longest possible SID should be employed in examinations. However, if a greater than standard SID is used then greater intensity of radiation would be required to produce the same film density. Therefore it is recommended that only standard SIDs should be used [1].

Shielding

The third radiation protection action relates to shielding. Shielding implies that certain materials (concrete, lead) will attenuate radiation (reduce its intensity) when they are placed between the source of radiation and the exposed individual.

We shall discuss four aspects of shielding in diagnostic radiology :

1. X-ray tube shielding

2. Room shielding

(a) X-ray equipment room shielding

(b) Patient waiting room shielding.

3. Personnel shielding

4. Patient shielding (of organs not under investigation)

X-ray tube shielding (Source Shielding)

The x-ray tube housing is lined with thin sheets of lead because x-rays produced in the tube are scattered in all directions. This shielding is intended to protect both patients and personnel from leakage radiation [1]. Leakage radiation is that created at the X-ray tube anode but not emitted through the x-ray tube portal. Rather, leakage radiation is transmitted through tube housing. Manufacturers of x-ray devices are required to shield the tube housing so as to limit the leakage radiation exposure rate to 0.1 R hr-1 at 1 meter from the tube anode [4]. AERB recommends a maximum allowable leakage radiation from tube housing not greater than 1mGy per hour per 100 cm2 [2].

Room shielding (Structural Shielding)

The lead lined walls of Radiology department are referred to as protective barriers because they are designed to protect individuals located outside the X-ray rooms from unwanted radiation There are two types of protective barriers.

(a) Primary Barrier: is one which is directly struck by the primary or the useful beam.

(b) Secondary Barrier: is one which is exposed to secondary radiation either by leakage from X-ray tube or by scattered radiation from the patient.

The shielding of X-ray room is influenced by the nature of occupancy of the adjoining area. In this respect two types of areas have been identified.

(a) Control Area: Is defined as the area routinely occupied by radiation workers who are exposed to an occupational dose. For control area, the shielding should be such that it reduces exposure in that area to <26 mC/kg/week.

(b) Uncontrolled areas: Are those areas which are not occupied by occupational workers. For these areas, the shielding should reduce the exposure rate to <2.6mC/kg/week [1].

AERB has laid down guidelines for shielding of X-ray examination room and patient's waiting room which are as follows.

(a) X-ray examination room shielding

Rooms housing diagnostic X-ray units and related equipment are located as far away as feasible from areas of high occupancy and general traffic, such as maternity and paediatric wards and other departments of the hospital that are not directly related to radiation and its use. The room housing an X-ray unit is not less than 18 m2 for general purpose radiography and conventional fluoroscopy equipment. In case the installation is located in a residential complex, it is ensured that (i) wall of the X-ray rooms on which primary X-ray beam falls is not less than 35 cm thick brick or equivalent, (ii) walls of the X-ray room on which scattered X-rays fall is not less than 23 cm thick brick or equivalent, (iii) there is a shielding equivalent to at least 23 cm thick brick or 1.7 mm lead in front of the doors and windows of the X-ray room to protect the adjacent areas, either used by general public or not under possession of the owner of the X-ray room . Unshielded openings in an X-ray room for ventilation or natural light, are located above a height of 2 m from the finished level outside the X-ray room. Rooms housing fluoroscopy equipment are so designed that adequate darkness can be achieved conveniently, when desired, in the room.

Patient waiting area

Patient waiting areas are provided outside the X-ray room. A suitable warning signal such as red light and a warning placard is provided at a conspicuous place outside the X-ray room and kept 'ON' when the unit is in use to warn persons not connected with the particular examination from entering the room [2].

Shielding of the Xray control room :

The control room of an X-ray equipment is a secondary protective barrier which has two important aspects:

(a) The walls and viewing window of the control booth, which should have lead equivalents of 1.5mm.

(b) The location of control booth, which should not be located where the primary beam falls directly, and the radiation should be scattered twice before entering the booth [1].

The AERB recommends the following shielding for the X-ray control room: The control panel of diagnostic X-ray equipment operating at 125 kVp or above is installed in a separate room located outside but contiguous to the X-ray room and provided with appropriate shielding, direct viewing and oral communication facilities between the operator and the patient. In case of X-ray equipment operating up to 125 kVp, the control panel can be located in the X-ray room. AERB recommends that the distance between control panel and X-ray unit/chest stand should not be less than 3 m for general purpose fixed x-ray equipment [2].

The highly collimated X-ray beam in CT results in markedly non uniform distribution of absorbed dose perpendicular to the tomographic plane during the CT exposure. Therefore the size of the CT room housing the gantry of the CT unit as recommended by AERB should not be less than 25m2 [2].

Personnel shielding

Shielding of occupational workers can be achieved by following methods:

(a) Personnel should remain in the radiation environment only when necessary (step behind the control booth, or leave the room when practical)

(b) The distance between the personnel and the patient should be maximized when practical as the intensity of radiation decreases as the square of distance (inverse square law).

(c) Shielding apparel should be used as and when necessary which comprise of lead aprons, eye glasses with side shields, hand gloves and thyroid shields.

Lead aprons are shielding apparel recommended for use by radiation workers. These are classified as a secondary barrier to the effects of ionizing radiation. These aprons protect an individual only from secondary (scattered) radiation, not the primary beam [5]. The thickness of lead in the protective apparel determines the protection it provides. It is known that 0.25 mm lead thickness attenuates 66% of the beam at 75kVp and 1mm attenuates 99% of the beam at same kVp. It is recommended that for general purpose radiography the minimum thickness of lead equivalent in the protective apparel should be 0.5mm [5]. It is recommended that women radiation workers should wear a customized lead apron that reaches below midthigh level and wraps completely around the pelvis. This would eliminate an accidental exposure to a conceptus [6].

Care of the lead apparel: It is imperative that lead aprons are not abused, such as by dropping them on the floor, piling them in a heap or improperly draping them over the back of a chair. Because all of these actions can cause internal fracturing of the lead, they may compromise the apron's protective ability. When not in use, all protective apparel should be hung on properly designed racks. Protective apparel also should be radiographed for defects such as internal cracks and tears at least once a year [5].

Other protective apparel include eye glasses with side shields, thyroid shields and hand gloves. The minimum protective lead equivalents in hand gloves and thyroid shields should be 0.5mm.

AERB guidelines for personal protection

AERB has laid down recommendations for personnel protection of radiation workers. The protective barrier between the operator and X-ray tube should have a minimum lead equivalence of 1.5mm. Protective aprons and gloves should have a minimum lead equivalence of 0.25mm, and gonadal shields should have a minimum lead equivalence of 0.5mm. Any additional radiation protection devices which would be necessary for specialized radiological investigations should have a minimum of 0.5mm lead equivalence [2].

Patient shielding

Most radiology departments shield the worker and the attendant, paying little attention to the radiation protection of the patient. It has been recommended that the thyroid, breast and gonads be shielded, to protect these organs especially in children and young adults. In gonadal shielding, a lead apron is placed appropriately on the patient to protect the gonads from primary beam radiation exposure [1]. A lead bib and collar worn over the patient's neck and thorax have been documented to effectively shield radiosensitive organs like the thyroid and the breast, and are therefore recommended for routine use in dental X-rays and head CT examinations [7].

Radiation in the CT suite

It has been estimated that although CT accounts for less than 50% of all x-ray examinations it contributes upto 40% of the collective dose from diagnostic radiology [8]. CT Scanners have scattered radiation levels that may prove hazardous. The dose unit used in CT is the computed tomography dose index "CTDI". This measurement is defined in relation to the radiation field delivered at a specific point (x, y) by the CT Scanner. CTDI is usually expressed in terms of absorbed dose to air and is called CTDI air. Absorbed dose to tissue (Dtissue) is related to absorbed dose to air (Dair) by a mathematical coefficient which has a value of about 1.06 and an error not greater than ± 1%. Such measurements are made using a special pencil ionisation chamber or by a thermoluminescent dosimeter (TLD) [8]. Langer et al evaluated scattered radiation in a CT suite and documented that the radiation on the floor of the C.T. suite could be as high as 0.3 Gy/day. They concluded that adequate shielding should be provided for the floor and roof areas of a CT suite depending on which floor the CT is located. They proposed an additional thickness of 2.5mm of lead or 162mm of concrete to shield the front and rear reference points, so as to reduce the dose to 1 mgy/year [9].

Radiation Protection in CT suite

It is recommended that the radiologists who work in CT fluoroscopy (CTF) should employ lead glasses, thyroid shields, lead aprons, lead gloves, and portable body radiation barriers. They should wear both internal and external radiation badges to monitor their exposure to scattered radiation during the procedures. This allows the radiation levels associated with CTF to be measured so that radiologists can be judicious in selecting CT technique settings and in minimizing the fluoroscopic time for each procedure. Radiation exposure to patients and staff during CTF can be controlled by employing low mA settings, limiting imaging times, and employing appropriate radiation protection measures [10].

Radiation detection and measurement

The instruments used to detect radiation are referred to as radiation detection devices. Instruments used to measure radiation are called radiation dosimeters.

Methods of Detection

There are several methods of detecting radiation, and they are based on physical and chemical effects produced by radiation exposure. These methods are :-

1. Ionization

2. Photographic effect

3. Luminescence

4. Scintillation

Ionization : The ability of radiation to produce ionization in air is the basis for radiation detection by the ionization chamber. It consists of an electrode positioned in the middle of a cylinder that contains gas. When x-rays enter the chamber, they ionize the gas to form negative ions (electrons) and positive ions (positrons). The electrons are collected by the positively charged rod, while the positive ions are attracted to the negatively charged wall of the cylinder. The resulting small current from the chamber is subsequently amplified and measured. The strength of the current is proportional to the radiation intensity.

Photographic effect : The photographic effect, which refers to the ability of radiation to blacken photographic films, is the basis of detectors that use film.

Luminescence : Luminescence describes the property by which certain materials emit light when stimulated by a physiological process, a chemical or electrical action, or by heat. When radiation strikes these materials, the electrons are raised to higher orbital levels. When they fall back to their original orbital level, light is emitted. The amount of light emitted is proportional to the radiation intensity. Lithium fluoride, for example, will emit light when stimulated by heat. This is the fundamental basis of thermoluminescence dosimetry (TLD), a method used to measure exposure to patients and personnel.

Scintillation : Scintillation refers to a flash of light. It is a property of certain crystals such as sodium iodide and cesium iodide to absorb radiation and convert it to light. This light is then directed to a photomultiplier tube, which then converts the light into an electrical pulse. The size of the pulse is proportional to the light intensity, which is in turn proportional to the energy of the radiation[1].

Personnel Dosimetry

Personnel dosimetry refers to the monitoring of individuals who are exposed to radiation during the course of their work. Personnel dosimetry policies need to be in place for all occupationally exposed individuals. The data from the dosimeter are reliable only when the dosimeters are properly worn, receive proper care, and are returned on time. Proper care includes not irradiating the dosimeter except during occupational exposure and ensuring proper environmental conditions.

Monitoring is accomplished through the use of personnel dosimeters such as the pocket dosimeter, the film badge or the thermoluminescent dosimeter. The radiation measurement is a time-integrated dose, i.e., the dose summed over a period of time, usually about 3 months. The dose is subsequently stated as an estimate of the effective dose equivalent to the whole body in mSv for the reporting period. Dosimeters used for personnel monitoring have dose measurement limit of 0.1 - 0.2 mSv (10-20 mrem) [1].

Pocket Dosimeter

The pocket dosimeter monitors dose to personnel. It consists of an ionization chamber with an eyepiece and a transparent scale, as well as a hollow charging rod and a fixed and a movable fiber. When x-rays enter the dosimeter, ionization causes the fibers to lose their charges and, as a result, the movable fiber moves closer to the fixed fiber. The movable fiber provides an estimate of gamma or x-ray dose rate[1].

Film Badge Monitoring

These badges use small x-ray films sandwiched between several filters to help detect radiation. Film badges are inexpensive, easy to use, and easy to process. Although they are useful for detecting radiation at or above 0.1 mSv (10 mrem), they are not sensitive enough to capture lower levels of radiation. Their susceptibility to fogging caused by high temperatures and light means that they cannot and should not be worn for longer than a 4-week period at a stretch. Another major drawback to film badge monitoring is that it is an enormous task to chemically process a large number of small films and subsequently compare each to some standard test film[1]. In India, film badges have recently been replaced by TLD badges.

Thermoluminescent dosimetry (TLD) Monitoring

The limitations of the film badge are overcome by the thermoluminescent dosimeter (TLD). Thermolumine-scence is the property of certain materials to emit light when they are stimulated by heat. Materials such as lithium fluoride (LiF), lithium borate (Li2B4O7), calcium fluoride (CaF2), and calcium sulfate (CaSO4) have been used to make TLDs.

When an LiF crystal is exposed to radiation, a few electrons become trapped in higher energy levels. For these electrons to return to their normal energy levels, the LiF crystal must be heated. As the electrons return to their stable state, light is emitted because of the energy difference between two orbital levels. The amount of light emitted is measured (by a photomultiplier tube) and it is proportional to the radiation dose.

The measurement of radiation from a TLD is a two-step procedure. In step 1, the TLD is exposed to the radiation. In step 2, the LiF crystal is placed in a TLD analyzer, where it is exposed to heat. As the crystal is exposed to increasing temperatures, light is emitted. When the intensity of light is plotted as a function of the temperature, a glow curve results. The glow curve can be used to find out how much radiation energy is received by the crystal because the highest peak and the area under the curve are proportional to the energy of the radiation. These parameters can be measured and converted to dose[1].

Whereas the TLD can measure exposures to individuals as low as 1.3µC/kg (5 mR), the pocket dosimeter can measure up to 50 µC/kg (200 mR). The film badge, however, cannot measure exposures < 2.6 µC/kg (10 mR) . TLDs can withstand a certain degree of heat, humidity, and pressure; their crystals are reusable; and instantaneous readings are possible if the department has a TLD analyzer. The greatest disadvantage of a TLD is its cost[1].

Wearing the dosimeter

(a) During Radiography

During radiography (when no protective lead apron is worn), the personnel dosimeter is worn at one of two regions :

1. on the trunk of the body at the level of the waist, on the anterior side of the individual,

or

2. on the upper chest region at the level of the collar area on the anterior surface of the individual.

At these positions, the dosimeter readings represent an estimate of exposure at two different levels ie the whole body exposure is estimated by the trunk level badge and exposures dose to internal organs like thyroid is measured by the collar level badge [1].

(b) During Fluoroscopy

During fluoroscopy a protective apron should always be worn. It is further recommended that ideally two dosimeters should be worn by radiation personnel.

One at the collar level outside the lead apron and

the other at the trunk level underneath the lead apron.

The one at the collar level gives an accurate estimate of the radiation dose to the unprotected regions of head and neck. The dosimeter worn underneath the lead apron at the trunk level provides an accurate estimate of the radiation to the protected organs [1],[5]. If only one dosimeter is worn it must be worn at the collar outside the lead apron, because, the neck receives 10-20 times more radiation than the trunk which is protected by lead [5].

Radiation protection survey and programme

The responsibility for establishing a radiation protection programme rests with the hospital administration / owners of the X-ray facility . The administration is expected to appoint a Radiation Safety Committee (RSC), and a Radiation Safety Officer (RSO). It is recommended by NCRP that the RSC should comprise of a radiologist, a medical physicist, a nuclear medicine personnel, a senior nurse and an internist. It is the duty of RSC to perform a regular radiation protection survey. This survey has 5 phases which are:

2. Inspection: Each diagnostic installation in the department is examined for its protection status with respect to its operating factors, control booth and availability of protection devices.

3. Measurement: Measurements are conducted on exposure factors. In addition scattered radiation and patient dose measurements in radiography and fluoroscopy are performed

4. Evaluation: The radiation protection status of the department is evaluated by examination of records, equipment working, status of protective clothing and the radiation doses obtained from phase-3.

5. Recommendations: A report is prepared on the protection status of the department and the problem areas if any identified, for which recommendations are made regarding corrective measures[1][Figure - 3].

Surveillance of radiation workers

AERB has recommended regular medical examination of radiation workers to assess their protection status as per the following guidelines. "Every radiation worker prior to commencing radiation work and at subsequent intervals not exceeding 12 months shall be subjected to the following medical examinations:

1. X-ray examination of Chest.

2. All general laboratory investigations such as examination of blood and excreta.

3. Special investigations such as examination of skin, hands, fingers, nails and eyes."

Radiation Safety Officer

The NCRP has provided a brief description of the relevant qualifications and duties of an RSO. Every department should have an RSO. This Officer should be an individual with extensive training and education in areas such as radiation protection, radiation physics, radiation biology, instrumentation, dosimetry and shielding design [1].

In India AERB has specified duties of the RSO which include assisting the employer in meeting the relevant regulatory requirements applicable to his/her X-ray installation. He/she shall implement all radiation surveillance measures, conduct periodic radiation protection surveys, maintain proper records of periodic quality assurance tests, and personnel doses, instruct all workers on relevant safety measures, educate and train new entrants, and take local measures, including issuance of clear administrative instructions in writing, to deal with radiation emergencies. The RSO should also ensure that all radiation measuring and monitoring instruments in his/her custody are properly calibrated and maintained in good condition. The duties also include maintaining a record of all radiation surveys performed, deficiencies observed and remedial actions taken [2].

Recommended Dose Limits for Radiation Workers and General Public

The recommendations by ICRP, NCRP and AERB for occupational workers and general population are shown in [Table - 1][1],[2]:

Recommendations for dose limits to eye and skin for occupational workers are as shown in [Table - 2]:

Recommended Dose limits to Pregnant Women

The recommendations of various authorities are as follows:

1. ICRP : In pregnant females, a supplementary equivalent dose limit of 2mSv applied to the surface of her lower abdomen for the remainder of her pregnancy[1].

3. Aerb: Recommends that once pregnancy is established the equivalent to surface of pregnant woman's abdomen should not exceed 2 mSv for the remainder of the pregnancy [2].

Summary & Conclusions

Radiation protection is an integral component of the working infrastructure of any radiology department. The main principles of radiation protection are to provide adequate protection from undue exposure of radiation to personnel directly or indirectly involved with radiation, without unduly limiting the benefits of radiation exposure. The components of radiation protection include justification of the procedure involving the radiation exposure, use of minimum radiation exposure compatible with the procedure with provides adequate diagnostic information, shielding of the personnel and patient from unwanted radiation exposures and monitoring of radiation exposure to the occupational workers and the working environment. Regular surveillance of the department for radiation levels and monitoring of the radiation protection programmes and regular educational activities form an integral part of the responsibilities of the RSO and other administrative authorities of the department/hospital. The norms laid down by ICRP and AERB have to be followed in these surveys and protection programmes.